It is known that refinement of ferrite grain enhances strength and ductility of the steel products, a method for manufacturing a hot-rolled sheet having fine-grained ferrite has been an important art to develop function of the steel materials. In addition, since refinement of ferrite grain can enhance strength of the steel products without using specific (micro-alloying) elements, recycling rate of the products is high and burden over the global environment is less.
As a method for obtaining a hot-rolled sheet having the fine-grained ferrite, conventionally, large-strain deformation has been studied. For example, Patent document 1 discloses that high-strength hot-rolled sheet having fine-grained ferrite of carbon steel, whose grain size is 3 to 5 μm, can be obtained by single pass or under an accumulated large reduction at phase-transformation temperature region.
Moreover, Patent document 2 discloses that a fine-grained ferrite whose grain size is about 2-3 μm can be obtained by giving reduction at a rolling reduction of 40% or more within the temperature range of 650-950 degree C. and again giving continuous reduction within two seconds at a rolling reduction of 40% or more.
It is understood that these methods utilize a grain refinement mechanism with ferrite transformation and ferrite recrystallization during rolling.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 58-123823    Patent Document 2: JP-A No. 59-229413